Method of electroplating nickel on an aluminum article



United States Patent O 3,515,650 METHOD OF ELECTROPLATING NICKEL ON AN ALUMINUM ARTICLE Tahei Asada, Kobe, Japan, assignor to Kabushiki Kaisha Toyota Chuo Kenkyusho, Nagoya, Japan No Drawing. Filed May 19, 1967, Ser. No. 639,638 Claims priority, application Japan, June 2, 1966, 41/ 35,656 Int. Cl. C23b 9/00, 17/00 US. Cl. 204-42 Claims ABSTRACT OF THE DISCLOSURE A method for electroplating nickel on an aluminum article, comprising the steps of immersing the article as one electrode together with a second electrode in a plating bath formed of an electrolyte consisting of an aqueous solution of nickel salts, a pH value modifier and an electrical conductivity promoter, applying alternating current to the electrodes to produce a layer of aluminum oxide mixed with nickel oxide on the article, then applying direct current to the electrodes with the aluminum articles as the cathode to reduce said oxides and deposit additional nickel to form a firmly adherent coating on the article. The article is then removed, washed and desirably heat treated to make the nickel coating even more firmly adherent.

The present invention relates to electroplating and is particularly directed to the electrodeposition of nickel directly upon objects and articles of aluminum and/or aluminum alloys.

The term aluminum article as hereinafter used in the specification and claims, is intended in a generic sense to include objects and articles made of pure aluminum, aluminum, of various commercial grades and aluminum base alloys. Aluminum alloys, whether cast or wrought, are included, and whether of the heat-treatable or non heat-treatable types.

Conventional processes for nickel plating aluminum fall within three categories, as follows:

(1) A process which first deposits a zinc layer upon an aluminum article by immersion, then deposits a copper on said zinc layer by electrodeposition, and firmly elec troplates nickel on siad copper layer.

(2) A process which first anodizes the aluminum article and then electroplates nickel on the coating produced by the anodization.

(3) A method of nickel plating aluminum by electrodeless plating.

Each of these three methods has inherent difficulties and each is complicated by the necessity for repeated steps of etching by acids and alkalis of various kinds as well as water washing. In addition, the selection of treating solutions, including etching and washing, and combination of treating processes must be changed in accordance with the kind of base aluminum metal being plated.

The primary object of the invention is to provide a method of nickel plating aluminum which overcomes the disadvantage of conventional methods briefly outlined above.

An important object of the invention is to provide a simple and inexpensive method for electroplating nickel directly onto an aluminum article in a manner which does not require close controls, and which affords a resulting product in which the plated nickel is tightly adhered directly to the immediately underlying aluminum article.

A further imporatant object of the invention is to provide a method of electroplating nickel on aluminum which eliminates the need for complicated etching and washing steps and avoid the need for changing the treating solutions in correspondence with changes in the aluminum base material being plated.

These, and other objects and advantages of the invention will become apparent from the following detailed description.

The present invention provides a method of electroplating nickel on aluminum, wherein a cleaned and degreased aluminum article is first electrolyzed by alternating current, and subsequently by direct current in the same plating bath.

The aluminum article is initially cleaned and degreased simply by using organic solvents or a soapless detergent. The cleaned and degreased aluminum article and a nobler metal, such as stainless steel, carbon or nickel, are placed as electrodes in an acidic plating bath consisting of an aqueous solution of nickel salts, a pH value modifier and an electrical conductivity promoter, and alternating current of several volts is applied for several minutes. In consequence of such alternating current electrolysis, nickel oxide is deposited in mixture with aluminum oxide on the surface of the aluminum article. After this alternating current electrolysis, the power source is immediately changed, and direct current with density of 0.2 to 20 ampere per square decimeter is applied to the electrodes, the aluminum article being the cathode. During this step of direct current electrolysis, the nickel oxide of the previously deposited mixture of oxides is reduced to nickel, and additional nickel is deposited from the nickel ions in the plating bath. Consequently, a heavy desired layer of nickel is obtained on the aluminum article. After the aluminum article has been nickel plated and removed from the plating bath, it is washed with water and dried. It may then be heat treated to obtain firmer adhesion between the aluminum article and the nickel layer.

In the conventional anodizing nickel plating method, (2 above), the adhesiveness of the final nickel plating is reduced because the oxidized layer on the aluminum article obtained by pre-treatment becomes less firmly adhered in the course of water washing and in the course of transferring the article to the plating bath after the pre-treatment. But in the method according to this invention, a strong and directly adherent layer is obtained without being weakened, because the step of depositing a nickel layer by direct current electrolysis can be practiced in succession to the step of depositing an oxidized layer by alternating current electrolysis in the same plating bath by simply changing the power source, intermediate washing and transferring to a different plating bath being omitted.

It should be noted, however, that if desired, the two steps of alternating and direct current electrolysis can be performed in separate baths in this invention. This would merely involve transferring the aluminum article into a nickel plating bath for direct current electrolysis after an oxide layer is first produced by alternating current electrolysis in an anodizing bath.

The electrolyte used in the present invention differs from the conventional anodizing bath in that it contains a very large quantity of nickel ions. Therefore, the nickel ions are deposited as nickel when the aluminum article is made the cathode, and oxidized nickel is deposited simultaneously with the formation of the oxidized aluminum layer when the aluminum article is made the anode.

Generally it has been known that the conventional oxidized layer produced on the surface of aluminum material by electrolysis is characterized by an aggregate of innumerable cells, each having fine pores of approximately 0.01 to 0.1 in diameter. Consequently, the conventional oxidized layer has innumerable small pores in it. Between portions of such cell structure and the base metal, there exists a so-called barrier layer having a rectifying effect similar to that of a p-n junction of a semiconductor. During the growth of an oxidized layer, the electric current flows almost exclusively through said fine pores but hardly at all through other portions of the oxidized layer.

This principle is applicable to a phenomenon of the present invention. That is, in the oxidized layer of aluminum, the electric current flows through the fine pores of said layer and causes deposition of nickel in the inner surfaces of the fine pores, and the nickel oxide is produced in such manner that it is wedged in the fine pores of aluminum oxide. It is, therefore, possible to make the adhesion of the final nickel layer much firmer, as the nickel layer is produced by reducing the nickel oxide deposit and by depositing nickel from nickel ions in the electrolyte directly upon the reduced nickel.

In conventional methods, all requiring etching of the surface of the base metal before electroplating so as to secure a desired adhesion, the resultant plated surface is dull in appearance, which makes it necessary to finally bull and polish after electroplating, or in course of electroplating to obtain a desired brightness.

The present invention does not require etching, but merely an initial degreasing of the aluminum article and this is enough to provide a desired surface for nickel plating with tight adhesion of the nickel layer.

The nickel plating of the present invention can be used as an excellent undercoating for other plating, such as bright nickel and chromium, providing tight adhesion for the later depositions.

The environment and conditions for practicing the present invention are as follows. The electrolyte primarily may comprise one or more kinds of nickel salt, such as nickel sulphate, nickel ammonium sulphate, or nickel chloride, to which is added one or more of the following: boric acid, urea, ammonium sulphate and ammonium acetate for limiting the transfer of pH value accompanying the plating operation, and to which is further added one or more of the following salts: magnesium sulphate, ammonium sulphate, or sodium sulphate for increasing electric conductivity. In practice, the electrolyte is acidized for use. The pH value of the plating bath varies during the plating operation, but its change will not obstruct the formation of the nickel layer. It is desirable, however, to maintain a constant pH value of the electrolyte for securing a stable operation.

A counter electrode to the aluminum article can be chosen as in conventional plating methods. To compensate for decrease in nickel ions caused by deposition the counter electrode can be formed of nickel, or if compensation is achieved by adding nickel salt to the electrolyte, the electrode can be insoluble such as carbon or stainless steel. In the latter case, any other nobler metal than aluminum may be used as a counter electrode. The alternating current electrolysis may be practiced approximately at 2 to 10 volts for 1 to 10 minutes, and the direct current electrolysis at a current density of cathode between 0.2 to 20 amperes per square decimeter for 10 to 13 minutes.

The resultant nickel plating normally has sufficient adhesion but, if desired or necessary, the adhesion can be increased and made much firmer if the article is heat treated at a temperature of 100 C. (212 E), for 15 minutes. The time of this heat treatment may be shortened by raising the temperature, for example, the treatment may be at a temperature of 300 C. (572 F.), for a period of approximately 5 minutes.

EXAMPLE 1 Nickel ammonium sulphate 89 gr. (2.82. oz.), magnesium sulphate 50 gr. (1.76 oz.), and boric acid 25 gr. (0.88 oz.), are dissolved in 1 liter (61 cu. in. of water. The initial pH value of the aqueous solution is prepared to be about 4.5 to 5.0. The electrolyte is maintained at a temperature of 45 C. (113 F.), and in the bath are placed an aluminum plate (99.7% in purity) cleaned on the surface and a nickel plate spaced apart a distance of about 5 cm. (1.97 in.). Alternating current at a voltage of 6 v., is applied to the plates, or electrodes, for a period of about 5 minutes. During this electrolysis, aluminum oxide is produced on the surface of the aluminum plate and simultaneously nickel oxide is deposited thereon. The current is then changed to direct current, and the electrolysis continued for about 13 minutes with the aluminum plate as the cathode at a voltage of 3 v., and a current density of 1.5 amperes per square decimeter, resulting in that the nickel oxide is reduced and nickel is deposited. The aluminum plate with nickel plating thereon is then removed from the bath, washed and dried. If desired, the plated aluminum article is further heat treated for several minutes at a temperature of about 300 C. (570 E), which Will increase the adhesion of the nickel layer to the aluminum plate.

EXAMPLE 2 The plating bath is formed by dissolving nickel sulphate 200 gr. (14.1 oz.), nickel chloride 10 gr. (0.35 oz.), urea 50 gr. (1.76 oz.), and boric acid 25 gr. (0.88 oz.), in 1 liter (61 cu. in.), of water. The aqueous solution is prepared to yield a pH value of about 4.5 to 5.0. The plating bath is maintained at a temperature of 45 C. (113 F.). Cleaned 63S (ALCOA Standard) wrought alloy as the anode and a carbon plate as the cathode are immersed in the plating bath spaced apart a distance of about 5 cm. (1.97 in.). An alternating current voltage of 5 v., is applied for a period of about 3 minutes. During this period a layer formed of a mixture of nickel oxide and aluminum oxide on the surface of the aluminum alloy is deposited. Next, the alternating current is replaced by direct current with the aluminum alloy as the cathode and a nickel plate as the anode. The electrolysis is con tinued at a voltage of 3 v., and a current density of 2.0 amperes per square decimeter for about 10 minutes, whereby the nickel oxide is reduced and additional nickel is deposited on the surface of the resultant nickel layer on the aluminum alloy. After washing and drying the product is heat treated at a temperature of about 200 C., (390 F.), for several minutes to further improve the adhesiveness of the plated layer.

EXAMPLE 3 An electrolyte of the same composition described in Example 1 is employed, i.e., comprising nickel ammonium sulphate gr. (2.82 oz.), magnesium sulphate 50 gr. (1.76 oz.), and boric acid 25 gr. (0.88 oz.), dissolved in 1 liter (61 cu. in.), of water. The solution is prepared to have a pH value of about 4.5 to 5.0. The electrolyte, while maintained at a temperature of 45 C. (113 F.), has placed therein a carbon plate as an anode and a surface cleaned 2S (ALCOA Standard) aluminum plate as a cathode, these electrodes being spaced apart a distance of about cm. (1.97 in.). An alternating current voltage of 6 v., is applied to the electrodes in the plating bath for about 5 minutes. There is produced a layer of aluminum oxide and nickel oxide in mixture on the surface of the 28 aluminum plate. This plate with layer product is transferred as a cathode into a plating bath whose electrolyte has the composition stated in Example 2, above, i.e., nickel sulphate 200 gr. (14.1 oz.), nickel chloride gr., (0.35 02.), urea 50 gr. (1.76 02.), and boric acid 25 gr. (0.88 02.), which are dissolved in 1 liter (61 cu. in.), of water. The solution is initially prepared at a pH value of about 4 .5 to 5.0. In the electrolyte, there is immersed in addition to the 28 aluminum plate as the cathode, an electrolyzed nickel plate as the anode and spaced apart a distance of about 5 cm. (1.97 in.). Direct current of a density of 1.5 amperes per square decimeter is passed through the electrolyte for about 13 minutes at room temperature. The 28 aluminum plate is removed, washed and dried, and then heat treated for several minutes at a temperature of about 200 C. (390 F.). The aluminum plate will then have the desired nickel layer.

In all of the examples described above the initial pH value of the plating .bath has been stated to be about 4.5 to 5. 0. It should be noted that the initial value need not be constantly maintained during plating as satisfactory results have been obtained with the bath being acidic (e.g. having a pH value below *6) and good results have been obtained experimentally with a bath having a pH value as low as 1.3.

Although certain specific embodiments of the invention have been described, it is obvious that many modifications thereof are possible. The invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

What is claimed is:

1. A method for the electrodeposition of nickel onto an aluminum surface taken from the group consisting of aluminum and alloys thereof comprising the steps of:

(a) providing an electrolytic cell containing an electrolyte consisting of an aqueous acid solution including a source of nickel ions, a pH modifying agent and an electrical conductivity promoter,

(b) immersing a first and second electrode into said electrolyte, said first electrode providing the aluminum surface to be nickel plated,

(c) electrolytically oxidizing said first electrode surface and producing a layer of aluminum oxide comixed with nickel oxide thereon, said comixed oxide layer being formed by applying an alternating current at a voltage from about 2 to about 10' volts across the electrodes for an elfective period of time from about 1 to about 10 minutes,

(d) electrolyticaly reducing said comixed oxides formed in step (c) to metallic states by first terminating said alternating current and thereafter applying a direct current across said electrodes at a cathode current density ranging from about 0.2 to about amperes per square decimeter for an effective period of time from about 10 to about 13 minutes, said first electrode being made the negative electrode with respect to the second electrode, and

(e) simultaneously with step (d) electrolytically depositing nickel metal from said electrolytic solution to said first electrode surface forming tenaciously adhering nickel plate.

2. A method in accordance with claim 1, wherein said electrolyte is initially adjusted to a pH value below about 6.

3. A method in accordance with claim 2, wherein the second electrode is formed of a material taken from the group consisting of carbon and metals nobler than aluminum.

4. A method in accordance with claim 3, wherein said electrolyte comprises nickel salts taken from the group consisting of nickel sulphate, nickel ammonium sulphate, nickel chloride, and mixtures thereof; said pH modifier is taken from the group consisting of boric acid, urea, ammonium sulphate, ammonium acetate, and mixtures thereof; said electrical conductivity promoter is taken from the group consisting of magnesium sulphate, ammonium sulphate, sodium sulphate, and mixtures thereof.

5. A method in accordance with claim 4, wherein the steps enumerated in (c), (d), and (e) of claim 1 are performed using the same electrolyte.

6. A method for electroplating nickel on an aluminum article comprising the steps of:

(a) effectively cleaning the aluminum article to be plated prior to its electrolytic treatment,

(b) providing means for electroplating including at least one electrode formed from the aluminum article and a second electrode taken from the group consisting of stainless steel, nickel and carbon, an electrolyte consisting essentially of an aqueous acid solution comprising a nickel salt taken from the group consisting of nickel sulphate, nickel ammonium sulphate, nickel chloride and mixtures thereof; a pH modifier taken from the group consisting of boric acid, urea, ammonium sulphate, ammonium acetate, and mixtures thereof; an electrical conductivity promoter taken from the group consisting of magnesium sulphate, ammonium sulphate, sodium sulphate, and mixtures thereof,

(0) electrolytically oxidizing said first electrode surface and producing a layer of aluminum oxide comixed with nickel oxide thereon, said comixed oxide layer being formed by applying an alternating current at a voltage of from about 2 to about 10 volts across the electrodes for an effective period of time from about 1 to about 10 minutes,

(d) electrolytically reducing said comixed oxides formed in step (c) to metallic states by first terminating said alternating current and thereafter applying a direct current across said electrodes at a cathode current density ranging from about 0.2 to about 20 amperes per square decimeter for an effective period of time from about 10 to about 13 minutes, said first electrode being made the negative electrode with respect to the second electrode, and

(e) simultaneously with step (d) electrolytically depositing nickel metal from said electrolytic solution to said first electrode surface forming tenaciously adhering nickel plate.

7. A method in accordance with claim 6, wherein the electrolytic oxidation and reduction steps are performed in the same electrolytic cell using the same electrolyte.

8. A method in accordance with claim 6, wherein the electrolytic oxidation step is performed in a first electrolytic cell and the electrolytic reduction step is subsequently performed in a second electrolytic cell.

9. A method in accordance with claim 6, wherein a heat treatment is subsequently applied to the plated article and comprises heating at an elevated temperature for a period up to about 15 minutes.

10. A method in accordance with claim 9, wherein said heat treatment step comprises applying heat at a temperature of 300.C., for a period of about 5 minutes.

(References on following page) Bengston, H., Trans. Electrochem. Soc. vol. 88, 1945,

References Cited UNITED STATES PATENTS Travers 204--33 Travers 204-42 Frasch 20433 McDerrnott 204--33 McKay 20433 Richard 204-33 Withers 204.33

Atkinson 20449 Asada 20458 FOREIGN PATENTS Canada.

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WINSTON A. DOUGLAS, Primary Examiner A. M. BEKELMAN, Assistant Examiner US. 01. X.P. 204-33, 3s 

